One of my biggest pet peeves as a Forensics Scientist is reading a story where the Forensics are completely wrong. I can overlook a lot of things, but bad science is not one of them.
You can use Google to find most everything in the science world. Make a few phone calls and get the facts straight. You can even embellish them for your story if the science is correct before you embellish.
Here are a few examples of bad science.
1. He lifted the revolver loaded with bullets he shot the man in the chest. A cartridge ejected, falling to the ground, making a clinking noise against the tile.
A revolver doesn't eject anything.
A cartridge is made up of the bullet, or projectile, that fires from the gun, and the casing - the metal part of the cartridge. The casing is what makes the clinking noise.
Sometimes writers will say the spent bullet is on the ground. Spent casings are usually in plain sight, scattered on the ground/floor. Spent bullets, which are sometimes called slugs, are often harder to find and not usually in plain sight. They are often distorted and mangled from hitting objects before they end up at their intended target.
Okay, just so you know, to reload a revolver, you can do one round at a time, or you can use a speed loader. A speed loader will allow your character to reload his revolver quickly and efficiently. A revolver can only hold six shots. Just saying. (Speed loaders are specific to the revolver/ammunition. Keep that in mind.)
2. One of the worst examples I have read, as recently as a book published last year, was "a detective walked into the room. Chairs were overturned and the smell of cordite still lingered in the air."
Cordite went out with WWII.
SERIOUSLY!! People, this is stupid. Unless you walk in within a millisecond after the shot is fired and you have the snout of a Tennessee Blue Tick hound, you might get a whiff of the GUNPOWDER or PROPELLANT, but not likely.
You would be better off describing the stippling around the wound. Describing if there was gunpowder present on the skin or clothing. This would tell your readers whether the gunshot was close, intermediate, or distant contact.
Only on television can DNA be analyzed within an hour. Now if you have a RapidHIT machine, you can get results in under two hours.
This machine is new on the market, under four years. Not many know about this outside the industry. If you are going to write about anything you are not familiar with, just take the time to research it.
Blood itself can be tricky. There is an entire science behind blood spatter alone. There are certain things that you do just to calculate the Area of Convergence and the Area of Origin.
Get the movement of the cops right as well. No detective would ever pick up a piece of evidence from a crime scene before it was documented and photographed.
Your Crime Scene Techs are also going to use the right container for the evidence collected. Case in point: you don't put any kind of blood evidence in a plastic bag.
You will use paper sacks for any evidence that is wet or needs to breathe.
It's the little things that will make your story believable or have your reader going "that's so impossible."
You need to get these simple facts straight. Not only will you take your reader out of the story, but you will also thrust them out of the book entirely. Too many of these mistakes as a writer and you lose credibility.
Here are a few websites that will help you:
On my website, you can find even more. Check my Resources page.